lapazmike

A few days ago the front tire tube blew out just after unloading from car rack. Used the tear down as opportunity to install new tubes and tires. The new tubes (Continental) came with only a knurled stop nut, which I installed on exterior of rim.

The old (purchased new 7 months ago and installed/by at LBS staff) tubes had a hex nut AND a knurled stop/retainer nut. They threaded the hex nut as close as possible to tube and stem insert through the rim.

The blow out occurred on one of the bulges (one on each side of the valve stem). The rear tube exhibits the same two bulges. I visualize the hex nut holding the tube off the rim tape and air pressure (50 psi) distorting the tube to conform.

What is the proper use of the larger hex nut on the stem?

Camilo

You don't need any nuts on a stem, don't bother using. I have no idea what a hex nut is for, I've never seen one.

So you're saying this hex nut was inside the rim, between the tube and the inner surface of the rim where the valve goes through the hole?

Never heard of such a thing, but I've only been using Presta valves for 30-40 years. maybe others have.

fietsbob

Nut keeps your pump head from pushing the stem into the tire instead of going over the stem.

With glued on sew up tires they are absent..

If repeated shear forces cut the rubber around the stem,
I have used 2 ring nuts on either side of the rim to cure that..

there are older Utility French Presta tubes , have bought some there , decades back,
the stem is strengthened by clamping to Both sides of the rubber with
a oval made for the purpose compression washer , and a nut, holding
the stem firmly in the tube.

It is a system advantaging low inflation pressures, as when tire is low ,
it creeps around the rim with torque , carrying the tube with it.

Only vulnerability I found in a thorn resistant tube was the shear forces of the tube shifting,
but the stem, thru the hole in the rim won't, and a leak around the vulcanized in stem occurs.
Keeping the tire topped up all the time
or gluing one bead to the rim , if low pressures are desired, will help.

rydabent

As stated the knurled nut keeps the stem from pushing into the wheel when you pump it up. I keep one on the bike for that purpose. It is also handy when changing tubes, it keeps the tube in place which helps keep the stem centered.

Al1943

This sounds like a bad installation job. When installing a tube and tire it is important to make sure no part of the tube is not caught between the tire bead and any part of the rim. This is a common problem adjacent to the valve stem. The tube is thicker at the base of the valve stem and can interfere with the tire bead. If the outside nut is used to tighten the valve against the rim before the tire bead is set in place it will cause the bead to not set and the tube will find a way to work into the gap in the bead resulting in a blowout. The way to avoid this is to push the valve back into the tire to make room for the bead to seat before tightening the nut.

The primary purpose of the nuts is to stabilize the valve and keeping it from rocking back and forth when using a frame pump. Many experienced riders (including me) do not use the nuts. I prefer tubes that have no threads on the valve stems because the threads cause the pump "O" rings to wear faster. And I don't use a frame pump either.

lapazmike

Camilo: Exactly.

fietsbob: Their is a concave washer under the hex nut compressed firmly to the tube.

Al1943 The blowout and bulges are on the Inside Diameter of the tube that would be in the center where the spokes atttach; Not radially from the stem as if it was a seating problem of the tire bead.

Are there many tubes on the market (USA & Latin America) that use a mechanical (washer and compression washer)
fastener of the stem to tube? The new tubes (Continental) appear to be glued or vulcanized.

pmt

Rule #60 / Ditch the washer-nut and valve-stem cap.

You are not, under any circumstances, to employ the use of the washer-nut and valve-stem cap that come with your inner-tubes or tubulars. They are only supplied to meet shipping regulations. They are useless when it comes to tubes and tires.

RegHarris

I just took a punctured tube (Presta valve) off my wheel & found a nut on the valve on the INSIDE of the
wheel rim. Never seen this before. This tube was installed at a bike shop.

cpach

Hey Reg, please don't post to 7 year old threads. It's confusing. Thanks. Also there shouldn't be a nut on the inside of the valve. They were perhaps trying to adapt a presta tube onto a shraeder rim but there are spacers sold for that specific purpose.